Why on-base percentage? For those in standard rotisserie or points leagues, why do you care about on-base percentage? Because in most roto formats, runs are a category, and you can't score without getting on base. Stolen bases are a category, and you can't steal without getting on base (cut to frustrated Austin Jackson owners).
These are not necessarily sleepers to have amazing, breakout years. These are guys who are being drafted in the mid to late rounds and will get on base to help you in multiple ways.
Some of these players are going as late as they are due to poor 2011 seasons, and they wouldn't be listed here if I didn't expect some level of bounce back, at least enough to make their current draft position a legitimate bargain.